Optical fiber communication offers many advantages over conventional wire based systems, these advantages including reduced losses, increased bandwidth, immunity from electromagnetic interference (EMI), and a high level of security. The application of optical fiber technology into the local area network (LAN) is, therefore, of increasing interest. In the past, however, it has been assumed that optical networks will only penetrate small business and residential sectors if new broadband services are provided to offset the additional costs involved in the installation of the optical technology. Some of the broadband services that could be provided are alpha-numeric videotex (e.g. Prestel), photographic videotex, high definition television, interactive video on demand (video library), video telephony, interactive graphics and high-speed data services.
Although the importance of providing such services has been recognised for some time, it is difficult for telecommunications operating companies to predict their market potential and therefore justify a major investment. What is required is an entry strategy that allows optical technology to be installed economically for telephony and low-speed data services, while maintaining the potential for evolution at a marginal cost for future broadband services.
In known optical networks, routing of information is achieved at each node by electronic means, that is to say by detecting the received optical signal to give an electrical signal (plus detector noise). This electrical signal must be regenerated, after processing and switching to remove the noise, before the signal is re-transmitted optically. Regeneration is bit-rate dependent, and so restricts the information type that can be carried, thereby preventing the transmission of broadband services. The need for regeneration could be removed by coupling off, at each node, part of the received optical signal, the coupled-off signal being converted to an electronic signal which is electronically processed, the remaining uncoupled optical signal being re-routed by the electronic processor. Unfortunately, the electronic processing times severely limit the possible capacity of the optical links, so again the provision of broadband services is not practical. Thus, although the electronic processor can switch quickly (of the order of nanoseconds) it requires a relatively long time (of the order of microseconds) to process, and therefore to decide upon the necessary route of the signal. In this scheme, the uncoupled optical signal is delayed during the processing time by a long length of optical fiber, and this obviously increases the size of each switching node.
Optical routing of information at the nodes of such an optical network would increase the capacity of the network by reducing the processing time. Not only would this increase the capacity of the network, it would also decrease the vast delay lengths of optical fiber otherwise required. Optical signal processing is well known, but the particular method of optical routing in a given network will depend upon the nature of that network. A particularly advantageous type of optical network is the recently developed telephony over passive optical networks (TPON). This type of network has no routing mechanisms, that is to say all terminals receive all the information transmitted by the exchange. One of the main advantages offered by TPON is the ability to move transmission between customers. This is because the gross bit-rate used with TPON is 20 Mbit/s (chosen mainly to allow cheap CMOS realisation of signal processing chips), and this is divided into a basic granularity of 8 kbit/s, that is to say 8 kbit/s is the basic transmission unit that can be moved from customer to customer (or can be summed to provide channels of nx8 kbit/s capacity). This ability suggests that TPON will be particularly applicable to the small business sector. TPON also shows great promise for the economic provision of optical fiber to the telephony customer, with major potential for later extension to broadband integrated services digital networks (ISDN).
In order to enhance management and flexibility of the core of the network of the telecommunications network, a synchronous digital hierarchy (SDH) managed transmission network is planned as a replacement for the present asynchronous trunk and junction networks. An SDH network would have four different levels, with a passive optical network (PON) at the lowest (Access) level, and a high capacity routed network at the upper (Inner Core or Long Haul) level. The Inner Core level would benefit the most from optically-controlled routing, as this level requires the largest capacity. The increase in capacity required at the Access level (because of the addition of extra services) would, however, benefit from the use of optical routing. At the Access level, it is envisaged that there would be sixty-four access points to each node. It would, therefore, be possible to address each individual node by a series of code sequences, each code sequence allowing up to sixty-four permutations.
One method of implementing an SDH network, that achieves flexibility and supports the divergent needs of future services, is based on packet switching which is currently used in data networks where error-free communication is required. The protocols required for such a system contain sophisticated methods for correcting, retransmitting or re-routing packets, and so need a lot of processing which can cause long delays. To accommodate delay-critical, but error-tolerant services, such as voice, a much simpler protocol can be used to minimise the processing time required. An example of this technique, which is known as asynchronous transfer mode (ATM) is used for fast packet switching or asynchronous time division (ATD).
ATM is a label multiplexing technique that uses short, fixed length packets, or cells. The ATM cells are short to reduce packetisation delay, and are of fixed length to make it easier to bound delays through switches and multiplexers. They have short labels (or headers) to allow cells to be routed, at high speeds, by means of hardware routing tables at each switch. For large transmission bandwidths (.about.1Gbit/s or more) this routing may be most effectively performed optically via optical code recognition (OCR).
The packet header and information fields must be separated at nodes where OCR of the header is to take place. This could be achieved by having the information field at bit-rates far in excess of the header bit rate and the response time of the optical code recognition unit (OCRU), so that the OCRU, being too slow to "see" the information field bit rate will only "see" a constant intensity after the header. Alternatively, and preferably, the header and information fields could be at different wavelengths, so that they may be split easily, either by a wavelength dependent coupler or by means of wavelength division multiplexing technology.
In developing a system of optical code recognition for use in optical routing of TPON, the following requirements must be met, namely:
(a) Around 64 codes are required with the minimum of redundancy. This is due to the SDH network requiring up to 64 codes at each level of the network adequately to address each access terminal; PA1 (b) The OCRU should be self timing, that is to say a clock signal should not be required to synchronise the OCRU; PA1 (c) The OCRU should be realised using the minimum number of components, thus keeping cost and complexity down; PA1 (d) The match/mismatch decision of the OCRU must be achieved very quickly, that is to say the OCRU must have lower processing times than electronic systems; and PA1 (e) The logic level of the OCRU output should be kept to a minimum, since multiple level logic is easily degraded by the noise that is always present in real systems.
The specification of our International patent application GB 93/00090 describes an OCRU for recognising a predetermined n-bit optical code. The OCRU comprises an n-way splitter having an input and n parallel outputs, a plurality of combiners associated with the splitter outputs, and a respective gate controlled by the output of each of the combiners. Each of the splitter outputs is subject to a different delay of from 0 to (n-1) bit periods, and each combiner receives an input from at least one of the splitter outputs. The OCRU is such that all the gates are turned on if a predetermined optical code is applied to the splitter input. Each combiner is configured to operate at 2-level logic, and the arrangement is such that, when the predetermined optical code is input to the n-way splitter, each combiner receives an input of one or more `0`s or one or more `1`s, and each combiner receiving `1` inputs receives a maximum of two such inputs.
With this arrangement, each gate receiving one or more `1`s performs the `AND` logic operation, and each gate receiving one or more `0`s performs the `INVERTER` logic operation. The disadvantage of this is that, although `AND` logic operations can be implemented fairly easily in a number of technologies, for example by semiconductor based devices, fiber based devices (such as loop mirrors) or polymer devices, `INVERTER` logic operations are much harder to implement. Another disadvantage of this known split-and-combine technique is that two bits (the first and last) must be used for identifying the start and end of an input code sequence, so the code efficiency of the technique is reduced to 25%.